(1) Field of the Invention
The present invention relates to a process for firing ceramic honeycomb structural bodies.
(2) Related Art Statement
Ceramic honeycomb structural bodies have been obtained by mixing a ceramic raw material and a shaping aid or a pore-forming agent, extruding the thus obtained ceramic bodies to form green ceramic honeycomb structural bodies, and firing the obtained green ceramic structural bodies at specific temperatures in a given atmosphere inside a tunnel kiln or periodic kiln.
Since honeycomb structural bodies contain air in a number of through holes thereof, it is difficult to effectively heat the honeycomb structural bodies from the exterior thereof in the case of firing them in tunnel kilns or periodic kilns, even if the honeycomb structural bodies are light. When the honeycomb structural bodies contain a shaping aid such as graphite, wheat powder or starch powder or a shaping aid, e.g., an organic binder such as methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, starch paste or glycerin, a surface active agent, wax, or the like, such a shaping aid and a pore-forming agent do not burn easily. Therefore, it takes a long time to completely burn them off. Further, a time period during which such a shaping aid or a pore-forming agent burns away differs between the inner and outer side portions of the honeycomb structural body. Thus, they are not burnt uniformly over the inside and outside portions of the honeycomb structural body. Accordingly, the honeycomb structural bodies do not uniformly shrink, and thus often; crack. In some cases, the ceramic honeycomb structural bodies obtained may have a nonuniform porosity or pore diameter between the inside and outside portions thereof. Further, since the honeycomb structural bodies are not uniformly heated over the inside and outside portions, the inner and outer portions of the honeycomb structural body do not uniformly shrink so that the honeycomb structural bodies are partially dimensionally changed or are deformed.
Moreover, since heat is trapped inside numerous through holes of the honeycomb structural bodies even during cooling during the production it is difficult to cool the honeycomb structural bodies from the exterior thereof, thus prolonging a cooling time period.